In recent years, in optical discs of the Blu-ray Disc™ standard, an optical disc including three or four recording layers has been developed and standardized in order to increase the recording capacity. Moreover, it is expected that the development of a four-or-more recording-layer optical disc will be performed from now on with an objective of implementing even larger capacity. For example, in Non-Patent Literature 1, the description has been given concerning the following optical disc (i.e. grooveless disc): Namely, in this grooveless disc, there is provided a layer (which, hereinafter, will be referred to as a servo layer) that is equipped with a physical groove structure for performing the tracking servo control. Furthermore, there are provided layers for performing recording/reproduction (“recording layers”) that are equipped with none of the land/groove structure. It is considered that this grooveless disc is easy to fabricate even if a large number of recording layers are to be multilayered.
Also, in the abstract of Patent Literature 1, the disclosure has been made as follows: “An additional-writing start position is detected which is continuous to the formerly-recorded area in one of the recording layers of a guide-layer-separated-type optical recording medium. At the time of starting the additional recording, the irradiation spot of a servo-use first laser beam is displaced to the position on a guide track which is directly opposed to a position in the recording layer that is apart from the additional-writing start position onto the unrecorded-area side. The irradiation spot of a recording-or-reproduction-use second laser beam onto the recording layer is displaced in a follow-up manner by this first laser's irradiation-spot displacement. The additional recording into the recording layer is started from the irradiation-spot position of the second laser beam after this follow-up displacement is over.”